The present invention relates generally to thin-film electroluminescent (EL) display devices of the type having an inherent "memory", or voltage vs. brightness hysteresis. More particularly, the invention relates to an improved method of erasing such devices.
Reports of long service life and an inherent memory capability have focused increased attention on a.c.-driven ZnS:Mn thin film EL devices in recent years. The hysteretic behavior of the brightness vs. applied voltage relationship of such devices (which is responsible for the memory effect) was first described by Y. Yamauchi et al. in the 1974 International Electron Devices Meeting Digest, pp. 348-351. Since that time, memory-type ZnS:Mn thin film EL devices have been used in a variety of information display applications. For example, C. Suzuki et al. have published descriptions of X-Y matrix EL panels with inherent memory for displaying alphanumeric characters (1976 S.I.D. International Symposium Digest pp. 50, 51) and television pictures (Information Display, Spring, 1977, pp. 14-19). U.S. Pat. No. 4,149,108 to I. Chang discloses a multistable cathode-ray tube that uses an a.c. thin-film EL panel as the storage display target. The electroluminescent target of the Chang CRT is addressed directly by the tube's electron beam. U.S. application Ser. No. 922,950, filed July 10, 1978 in my name and assigned to the assignee of the present invention, describes an EL storage CRT with a target comprising a double insulated ZnS:Mn thin-film panel having a layer of a UV-emitting phosphor on the input side. Rather than being exposed to direct bombardment by the electron beam the EL panel is addressed by ultraviolet light emitted from the intermediate phosphor layer.
With all such devices there is a need to erase stored information. The conventional method of erasing an a.c. thin-film EL display panel is to reduce the amplitude of the applied a.c. signal, which is normally maintained at an image-sustaining level Vs, to a value below the extinction voltage Ve of the device. At or below Ve, the a.c. field across the thin-film EL layer is insufficient to maintain luminescence and the entire panel becomes dark, (unwritten). The erase cycle is completed by raising the applied signal potential to Vs, which returns the panel to an erased, ready-to-write condition.
Although it is easy enough to do, the above-described procedure has a significant drawback--a faint, residual image of the previously displayed information often remains when the applied voltage is returned to Vs. The residual image problem can be eliminated by adding a preliminary step to the erasing process: increasing the applied potential to a value Vw high enough to place the entire panel in a bright, fully-written condition. Then, when the a.c. voltage is dropped to Ve and returned to Vs, the panel will be fully erased and free of residual images. Unfortunately, however, the added step produces a momentary bright "flash" that is unpleasant and greatly increases visual fatigue. The effect is particularly objectionable with large size display panels, or displays used in low ambient light conditions.
Thus, there is a demonstrated need for an improved method of erasing memory-type electroluminescent display devices--one that is free from the drawbacks of the methods described above. A principal object of the present invention is to meet this need in a practical and satisfactory manner.
A more specific object of the invention is to provide a method for erasing memory-type EL panels that results in complete erasure of all previously-displayed information without increasing visual fatigue.
Another object of the invention is to provide an erasure method that is simpler and less costly to implement than prior art methods.
Still another object of the invention is to provide an improved method of total erasure that does not require any change in the amplitude of the a.c. voltage applied to the panel.
A further object of the invention is to provide an improved erasure method that is applicable in all memory-type thin-film EL panel applications.